The present invention is related to a system for measuring the acoustical attenuation effectiveness of devices employed to protect the hearing of workers subjected to relatively high levels of ambient noise.
Various types of hearing protection devices are in current use to protect the hearing of an individual working in an environment having a high ambient noise level. Generally, these hearing protection devices fall into two broad categories: earmuffs, which fit over and around the ears; and earplugs, which fit into the ear canal. Not only is there a substantial variation in the acoustical effectiveness of the different types of hearing protection devices, i.e., their ability to adequately attenuate the sound pressure level of certain hazardous noises to which the ear is exposed, but there is also a substantial variation in the effectiveness of single models of the same type, principally because of the variation in "goodness of fit." A slight defect in fit can render the protective device virtually ineffectual, thereby exposing without warning the ears of the wearer to hazardous ambient noise having frequencies and amplitudes sufficient to cause a gradual, painless and therefore often unnoticed permanent loss of hearing. All too often the ineffectiveness of a particular hearing protector is not detected until there has been a permanent impairment of the user's hearing.
At the present time, the most widely used means for monitoring the true effectiveness of hearing protection devices is to measure an individual's hearing level with a conventional audiometer at intervals of six months or more to determine if a permanent loss of hearing has occurred. Such audiometric monitoring, however, besides having the disadvantage of being "after the fact," is incapable of providing the information necessary to permit differentiation between those individuals who have sustained a hearing loss due to inadequate protection and those who have sustained losses due to other causes such as infectious disease, degenerative disease, and the like. This inability of present monitoring means to differentiate between work-caused hearing losses and those suffered from other causes has many medico-legal ramifications for both the worker who has suffered a hearing loss while working in a high-noise environment and for his or her employer.
Various means have been devised for objectively testing the acoustical effectiveness of muff-type hearing protectors. See, for example, Tegt U.S. Pat. No. 3,729,598 and a paper entitled "Method for the Measurement of Real-Ear Protection of Hearing Protectors and Physical Attenuation of Ear Muffs, ASA STD 1-1975" available from the Acoustical Society of America, 335 East 45th Street, New York NY 10017. The latter reference also includes a description of a method for subjectively testing the effectiveness of plug-type hearing protectors in a noise-free environment, such as found in a soundproof booth or test room, and employing a plurality of spaced loudspeakers to produce a free-field test signal.
The only presently accepted methods of testing ear protection devices, including the methods disclosed by the aforementioned paper of the Acoustical Society of America, require that the individual be placed in an ambient noise-free environment such as a soundproof booth and subjected to a series of free-field test tones at different frequencies and amplitudes to determine his acoustical threshold level with the hearing protection devices in place (occluded threshold) and his acoustical threshold level with the hearing protection devices removed (open threshold), and then comparing the occluded and open threshold level amplitude measurements to determine the acoustical attenuation provided by the hearing protectors. These methods, because they do not test each ear and thereby each of a pair of hearing protectors separately, while helpful in determining the attenuation provided by the worst of the two protectors or the protector positioned in or around the better of the subject's two ears, are not capable of indicating which of the protectors or which of the ears is being tested. Moreover the requirement for testing in an ambient-noise free environment requires physically removing the subject from his working environment which is time-consuming and therefore costly, and limits as a practical matter the frequency with which such tests can be conducted.
Furthermore, after the individual's occluded and open threshold levels have been measured at different frequencies and the attenuation provided by the hearing protector has been calculated for each frequency, the attenuation calculations must still be weighted according to the noise spectra of the work environment and the relative hazard of the different frequencies. These calculations and weighting are both complicated and time consuming.
Accordingly, the known prior art methods for measuring the attenuation provided by hearing protection devices are infeasible and uneconomic for general field use and must of necessity be restricted almost exclusively to use in the laboratory for developing data regarding the acoustical effectiveness of specific models of hearing protectors. No convenient method has yet been devised to measure the attenuation effectiveness of a particular hearing protector while it is in place in the ear of the user under normal conditions of fit at his workplace. Thus, even though a particular type of hearing protector may function effectively during a laboratory-type test, there is no guarantee that it will function effectively when worn by persons other than the individual tested in a laboratory and under other than laboratory-type conditions. Consequently, a worker who requires adequate noise protection against hazardous ambient noise has no way of knowing if his hearing protection is really failing until he has suffered a permanent measurable loss of hearing.